Int J Thermophys (2007) 28:1753–1765
DOI 10.1007/s10765-007-0253-4
Preparative Steps Towards the New Definition
of the Kelvin in Terms of the Boltzmann Constant
J. Fischer · S. Gerasimov · K. D. Hill · G. Machin ·
M. R. Moldover · L. Pitre · P. Steur · M. Stock · O. Tamura ·
H. Ugur · D. R. White · I. Yang · J. Zhang
Published online: 15 November 2007
© Springer Science+Business Media, LLC 2007
Abstract The International Committee for Weights and Measures (CIPM) approved,
in its Recommendation 1 of 2005, preparative steps towards new definitions of the
kilogram, the ampere, the kelvin, and the mole in terms of fundamental constants.
Within the Consultative Committee for Thermometry (CCT), a task group (TG-SI)
H. Ugur is the President, Consultative Committee for Thermometry (CCT).
J. Fischer (B)
Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany
e-mail: [email protected]
S. Gerasimov
D.I. Mendeleyev Institute for Metrology (VNIIM), St. Petersburg, Russia
K. D. Hill
National Research Council of Canada (NRC), Ottawa, Canada
G. Machin
National Physical Laboratory (NPL), Teddington, UK
M. R. Moldover
National Institute of Standards and Technology (NIST), Gaithersburg, MD, USA
L. Pitre
Conservatoire national des arts et métiers/Institut National de Métrologie (LNE-INM/CNAM),
La Plaine-Saint-Denis, France
P. Steur
Istituto Nazionale di Ricerca Metrologica (INRiM), Turin, Italy
M. Stock
Bureau International des Poids et Mesures (BIPM), Sevres, France
O. Tamura
National Metrology Institute of Japan, AIST (NMIJ/AIST), Tsukuba, Japan
H. Ugur
Teknoyad, Gebze-Kocaeli, Turkey
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has been formed to consider the implications of changing the definitions of the
above-mentioned base units of the SI, with particular emphasis on the kelvin and
the impact of the changes on metrology in thermometry. The TG-SI has presented the
results of its deliberations to the CCT and to the Consultative Committee for Units,
CCU, and worked with them to prepare a report to the CIPM. This contribution, authored by the members of TG-SI, solicits input from the wider scientific and technical
community on this important matter at the TEMPMEKO 2007 conference. For this
purpose, the main details of the report to the CIPM are presented. The unit of temperature T , the kelvin, can be defined in terms of the SI unit of energy, the joule, by fixing
the value of the Boltzmann constant k, which is simply the proportionality constant
between temperature and thermal energy kT . Currently, several experiments are underway to determine k. The TG-SI is monitoring closely the results of all experiments
relevant to the possible new definition of the kelvin, and has identified conditions to
be met before proceeding with the proposed redefinition. The TG-SI considers that
these conditions will be fulfilled before the 24th General Conference on Weights and
Measures in October 2011. Therefore, the TG-SI is recommending a redefinition of
the kelvin by fixing the value of the Boltzmann constant. A new definition of the kelvin
in terms of the Boltzmann constant does not require the replacement of ITS-90 with
an improved temperature scale nor does it prevent such a replacement.
Keywords Boltzmann constant · Fundamental physical constants · International
System of Units · Kelvin · Primary thermometry
1 Introduction
Thermometers that can be used to determine thermodynamic temperature directly are
few in number, difficult to employ, expensive, and not as precise or reproducible as
many practical thermometers. To meet the need for practical temperature measurement,
International Temperature Scales have been defined (ITS-27, IPTS-48, IPTS-68, ITS90, named after the year of promulgation) [1], which are essentially recipes for the
realization of highly reproducible and precise temperature standards which are in close
accord with the best thermodynamic measurements of the time. These scales have been
based on sets of fixed points, the defined temperatures of equilibrium states of certain
specified pure substances, and specified methods for interpolating or extrapolating
from these points [2].
Thus, the quantity determined in the vast majority of present-day temperature measurements is not thermodynamic temperature but T90 , as defined by the International
Temperature Scale of 1990, ITS-90 [3]. ITS-90 covers the range from 0.65 K to the
D. R. White
Measurement Standards Laboratory of New Zealand (MSL), Lower Hutt, New Zealand
I. Yang
Korea Research Institute of Standards and Science (KRISS), Daejeon, Korea
J. Zhang
National Institute of Metrology (NIM), Beijing, P.R. China
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highest temperature measurable in practice using the Planck radiation law. The ITS90 has recently been supplemented by the Provisional Low Temperature Scale from
0.9 mK to 1 K (PLTS-2000) [4], which covers the range from 0.9 mK to 1 K and defines
the quantity T2000 .
Recent developments in thermodynamic thermometry [5] have, for the first time,
offered primary thermometers with an accuracy which can approach or exceed the
precision of ITS-90, and with sufficient convenience to employ as standards. In principle, this makes it possible, for some temperature ranges at least, to dispense with
ITS-90 and measure true thermodynamic temperatures.
For many years, the CIPM has had the long-term aim of defining all of the base
units in terms of fundamental physical constants to eliminate any artifact or material
dependences and ensure the long-term stability of the units. In its Recommendation
1 of 2005, the CIPM approved preparative steps towards new definitions of the kilogram, the ampere, the kelvin, and the mole in terms of fundamental constants [6].
Within the CCT, the task group TG-SI considered the implications of changing the
definitions of the above-mentioned base units of the SI, with particular emphasis on the
kelvin.
For the kelvin, the change would generalize the definition, making it independent
of any material substance, technique of realization, and temperature or temperature
range. In particular, the new definition would improve temperature measurement at
temperatures far away from the triple point of water. For example, in the high temperature range, the radiometry community could apply absolute radiation thermometers
without the need to refer to the triple point of water. It would also encourage the
use of direct realizations of thermodynamic temperatures in parallel with the realization described in the International Temperature Scale. A new definition of the
kelvin in terms of the Boltzmann constant does not require the replacement of ITS-90
with an improved temperature scale, nor does it prevent such a replacement. In the
long term, it will enable gradual improvements to the temperature scale in respect
of lower uncertainties and extended temperature ranges, without the high transitional
costs and inconvenience that have been incurred with previous changes in temperature
scales.
This paper, authored by the members of TG-SI, provides background information
on the proposal and solicits input from the wider scientific and technical community
on this important matter at the TEMPMEKO 2007 conference.
2 Historical Background
The current definition of temperature was first suggested by William Thomson, later
Lord Kelvin, and is based on the amounts of heat entering and leaving an ideal heat
engine,
Qc
Qh
=
Th
Tc
(1)
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where Q h is the heat flowing into the engine from a hot reservoir at temperature Th ,
and Q c is the heat flowing out of the engine to a cold reservoir at temperature Tc .
Clausius observed that one consequence of Thomson’s definition is that the sum of
all of the heats flowing out of a heat engine multiplied by 1/T is identically zero for
reversible processes and greater than zero for irreversible processes, i.e.,
Qi
≥0
Ti
(2)
i
Clausius called the new quantity, S = Q/T , entropy, being a sort of transformed
energy (from the Greek trope meaning transformation).
The temperature defined by Thomson was based on the conceptual device of the
ideal heat engine; however, such a device is not necessary. The mathematician Carathéodory showed that, in any system, there exists a unique state variable that characterizes the reversibility of a process, and this variable is proportional to the integral
of the heats associated with the process divided by a unique “integrating factor” [7].
Comparison of Carathéodory’s result with Eq. 2 shows that the state variable is the
entropy defined by Clausius and the integrating factor is the temperature defined by
Thomson. Carathéodory’s result leads to a definition of temperature:
dS
1
=
T
dU
(3)
where U is the internal energy of the system and S is the conventional entropy.
Boltzmann subsequently showed that the entropy of a system is related to the
number of ways the constituent atoms and molecules can be arranged into the observed
macroscopic state:
S = kσ = kln P
(4)
where k is a constant, P is the probability of the system being in the observed state,
and σ is the entropy according to Boltzmann’s statistical definition. In his original
work, Boltzmann deduced the relation σ = ln P, without the factor k. This is in
line with the definition of entropy according to Shannon’s information theory [8],
and leads naturally to a thermodynamic temperature, τ , measured in energy units
(joule), 1/τ = dσ/dU , so, in principle, we do not need a separate base unit for
temperature, the kelvin. However, such a temperature scale would have impracticably
small (∼10−20 ) and unfamiliar values. For this reason, history went a different way
and Planck introduced the constant k, later named after Boltzmann, to provide the link
to conventional definitions of entropy and thermodynamic temperature [9].
Equation 3 applied to a variety of idealized systems yields thermodynamic relations
that can be used to measure temperature. For example, for an ideal gas, we can derive
the equation of state,
p Vm = NA k T
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where p is the pressure, Vm is the molar volume, and NA is Avogadro constant, the
number of particles per mole. Systems with equations of state that can be used to
measure temperature are often called primary thermometers (see Sect. 4 for further
examples).
The quantity kT = τ , which occurs in the equations of state, is a characteristic
energy determining the energy distribution among the particles of the system when
it is in thermal equilibrium. Thus, for unbound atoms, temperature is proportional
to the mean translational kinetic energy. Thermodynamic temperature is linear and
rational: equal intervals or ratios of temperature correspond to equal differences or
ratios of mean kinetic energy, and a single definition is required to fix the magnitude
of the temperature unit. All other temperature values must then be determined by
experiment, using a suitable thermal system and equation of state.
Today, the kelvin is defined in terms of the temperature of the triple point of water
and the Boltzmann constant k is a measured quantity. The CIPM proposal is to define a
numerical value for k, from which it follows that all temperatures, including the triple
point of water, must be measured. Of course, the adopted value for k will be such that
the temperature values will, as far as possible, remain unchanged.
3 Implications of the New Definition of the Kelvin
If the CIPM proposal is adopted, there will be a number of consequences for temperature measurement practice and the thermometry community. The most immediate
impact of the change is that it will endorse and encourage the use of both thermodynamic and ITS-90 temperatures. The change to a numerical definition of k will also
impact uncertainties in thermodynamic temperature measurements. In addressing the
consequences, the overriding factor is the need to minimize the transitional cost and
inconvenience to the measurement community, while gaining the benefits of improved
primary thermometry. This section summarizes some of the issues and how they may
be addressed.
3.1 Status of ITS-90
It is expected that the new definition for the kelvin will have little immediate impact on
the status of ITS-90. However, the ITS-90 will no longer be the only practical option
for temperature measurement. Thus, the most immediate and beneficial consequence
of the change is for temperatures below ∼20 K and above ∼1300 K where primary
thermometers may offer users a lower thermodynamic uncertainty than is currently
available with ITS-90. However, the ITS-90 will remain in use for the foreseeable
future as a precise, reproducible, and convenient approximation to thermodynamic
temperature.
The long-term consequence of the change is that, as the primary methods evolve
and achieve lower uncertainties, they will become more widely used and will gradually
replace ITS-90 as the basis of temperature measurement. As now, there will be a need
to indicate whether the measurements and uncertainties refer to T or T90 .
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Table 1 Defining fixed points of the ITS-90 with uncertainties u(T90 ) of the best practical realization in
terms of ITS-90 and uncertainties u(T ) of the thermodynamic temperature
1
2
Fixed point
T90 (K)
3
Cu
1357.77
15
60
60.1
Au
1337.33
10
50
50.1
u(T90 ) (mK)
4
u(T ) (mK)
5
u(Tk fixed ) (mK)
Ag
1234.93
1
40
40.1
Al
933.473
0.3
25
25.1
Zn
692.677
0.1
13
13.1
Sn
505.078
0.1
5
In
429.7485
0.1
3
3.11
Ga
302.9146
0.05
1
1.15
H2 O
273.16
0.02
0
0.49
Hg
234.3156
0.05
1.5
1.55
Ar
83.8058
0.1
1.5
1.50
O2
54.3584
0.1
1
1.00
Ne
24.5561
0.2
0.5
0.50
e-H2
≈20.3
0.2
0.5
0.50
e-H2
≈17.0
0.2
0.5
0.50
e-H2
13.8033
0.1
0.5
0.50
4 He
4.2221
0.1
0.3
0.30
5.10
u(Tk fixed ) is the uncertainty in the thermodynamic temperature of the listed phase transitions (which
presently serve as fixed points on ITS-90) assuming a new definition for the kelvin is adopted with a fixed
value for the Boltzmann constant. All values are quoted as standard uncertainties. Values in columns 3 and
4 have been taken from Table 1.2 of the Supplementary Information for the ITS-90 [2]
For the foreseeable future, most temperature measurements in the core temperature
range from about −200 to 960◦ C will continue to be made using standard platinum
resistance thermometers calibrated according to ITS-90. Because ITS-90 will remain
intact, with defined values of T90 for all of the fixed points, the uncertainties in T90
will not change: they will continue to be dominated by uncertainties in the fixed-point
realizations (column 3 of Table 1) and the non-uniqueness of the platinum resistance
thermometers, typically totalling less than 1 mK [10].
3.2 Uncertainties in Thermodynamic Temperatures
If the 2002 CODATA recommended value [11] of k = 1.380 650 5 × 10−23 J · K−1
were taken to be exact and used to define the kelvin, the relative uncertainty in k,
currently 1.8 × 10−6 , would be transferred to the temperature of the triple-point of
water, TTPW . This means that if such a new definition were to be adopted today, our
best estimate of the value of TTPW would still be 273.16 K, but instead of this value
being exact as a result of the definition of the kelvin as is now the case, the standard
uncertainty of the TTPW would be u(TTPW ) = 0.49 mK.
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Because all thermodynamic measurements are currently defined as ratios with respect to the triple point of water, the 0.49 mK uncertainty propagates to all historical thermodynamic temperature measurements. In practice, the change in definition
will only affect measurements made close to 273 K because the uncertainties of the
thermodynamic temperatures well away from this are very much larger than 0.49 mK.
To illustrate this point, columns 4 and 5 of Table 1 show for the defining fixed points
of ITS-90 the current [2] u(T ) values and the uncertainties when the value of the
Boltzmann constant will have been fixed.
The TG-SI could not foresee any experiment where the slightly increased uncertainties of thermodynamic temperatures u(Tk fixed ) would present a problem to metrology or the wider research community. It is also expected that any future changes in
the temperature scale will be much smaller than the tolerances associated with current documentary standards for thermocouples [12] and industrial platinum resistance
thermometers [13]. Therefore, no requirement is anticipated for any future change in
temperature scales to propagate to the documentary standards. Once the Boltzmann
constant has been fixed, which is expected to occur in 2011, the TG-SI is not aware
of any new technology for a primary thermometer providing a significantly improved
uncertainty u(TTPW ). Consequently, there will be no change of the assigned value of
TTPW for the foreseeable future.
The triple point of water will continue to have a role in practical thermometry. In fact,
the inconsistency of TTPW as realized by different triple-point-of-water reference cells
can be as small as 50 µK, or even smaller, if the isotopic composition of the water
used is taken into account [14,15]. Consequently, long-term experiments requiring
ultimate accuracy at or close to TTPW will still rely on the reproducibility of the triple
point of water. To overcome this situation, a determination of the thermodynamic
temperature TTPW would be required with an uncertainty smaller than about 50 µK.
This corresponds to a relative uncertainty in temperature of 2 × 10−7 .
3.3 The Mise en Pratique
To help users make accurate and reliable temperature measurements, the CIPM,
through its Consultative Committee on Thermometry (CCT) and the BIPM, is publishing a collection of guidelines for temperature measurement. This is similar to the
current Supplementary Information for the International Temperature Scale of 1990,
also published by the BIPM [2]. Following the practice established for length measurements, the guidelines are referred to as the mise en pratique of the definition of the
Kelvin (MeP) [16] and comprise recognized approximations to thermodynamic temperature currently including ITS-90 and PLTS-2000. The MeP will, in the future, be
expanded to describe recognized primary methods for measuring temperature or realizing the scale, and the sources of uncertainty associated with the measurements. The
MeP will be updated regularly as primary methods improve. Although there will be no
immediate changes to ITS-90, future revisions of the MeP will probably include improved approximations consistent with the best thermodynamic measurements. This
may be managed by revising ITS-90 and PLTS-2000.
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4 Proposals for the Wording of the New Definition
An “explicit-unit definition” explicitly defines a unit in terms of a particular quantity of
the same kind as the unit and, through a simple relationship implied by the definition
itself or one or more laws of physics, implicitly fixes the value of a fundamental
constant. Another option explicitly fixes the value of a fundamental constant and,
through a simple relationship implied by the definition itself or one or more laws of
physics, implicitly defines a unit; we call these “explicit-constant definitions.” The
explicit-unit definition of [17] for the kelvin is proposed as follows:
(1) The kelvin is the change of thermodynamic temperature that results in a
change of thermal energy kT by exactly 1.380 65X X × 10−23 joule.
Here, and in the following proposals, the XX will be replaced with the appropriate
digits of the Boltzmann constant when the new definition is established. The intention
of the redefinition is to move away from any material substance/artifact and base
the kelvin definition solely on a defined value for the Boltzmann constant. The new
definition of the kelvin will be realized by a wide variety of primary thermometers.
In the basic equations for all these thermometers appears the thermal energy kT [18].
Definition (1) is simple and intuitive and would clearly endorse any appropriate method
of measuring kT. However, the symbols k and T are undefined. Therefore, this slightly
more complicated form is preferred:
(1a) The kelvin is the change of thermodynamic temperature T that results in a
change of the thermal energy kT by exactly 1.380 65X X × 10−23 joule, where k is
the Boltzmann constant.
All of the SI base units implicitly define measurement scales that have natural zeros
and are therefore rational, i.e., all quantities can be expressed as ratios with respect to
the base unit, e.g., 273.15 K = 273.15×1 K. This is different on interval scales such as
the Celsius scale where 100◦ C is not equal to 100 × 1◦ C. On interval scales, the zero is
arbitrary and the proportionality constant must be expressed in terms of the derivative.
On a rational scale, it ought to be possible to express the proportionality constant
absolutely. For this reason, “change of” in the following definition has been omitted.
Moreover, a definition of the kelvin that relates to a gas is more easily understood, for
example, by a high school student, than any other definition:
(2) The kelvin is the thermodynamic temperature at which the mean translational kinetic energy of atoms in an ideal gas at equilibrium is exactly (3/2)
× 1.380 65X X × 10−23 joule.
Here, the broad but vague term “thermal energy” has been replaced by “mean translational kinetic energy.” At the same time, this definition avoids questions about the
kinetic energy associated with the internal degrees of freedom of a molecule by
introducing clearly atoms as the particles under consideration. By using “atoms” in
the plural, ensemble or time averages are included. However, the modifiers “ideal”
and “equilibrium” are required if we are to remain accurate, even though modifiers
interfere with clarity. This definition also includes the idea of an unbounded gas—but
if we include zero-point energy, then “change of” must be there. Certainly, quantized
systems need to include it, but an atom in an unbound (infinite volume) gas should
have no zero-point energy. However, the biggest problem with definition (2) is that
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it is essentially a “material” (i.e., gas)-based definition rather than a principle-based
definition.
To avoid the disturbing 3/2 factor, the number of degrees of freedom, which is
another complicating factor, has to be introduced:
(3) The kelvin is the thermodynamic temperature at which particles have an
average energy of exactly (1/2) × 1.380 65X X × 10−23 joule per accessible degree
of freedom.
This definition does cover both time and ensemble averages easily but still has problems, most notably the zero-point energy. The question is whether one needs to define
“particle” further—it cannot apply to all particles, e.g., photons, and how does it apply
more generally to particles of any mass or spin? In this definition, we specify the
number of degrees of freedom. This is the most general statement one can make.
Definitions 1–3 show how difficult it is to produce a satisfactory explicit-unit definition. Instead of being so specific, one could leave the definition sufficiently wide
to encompass any form of primary thermometry and leave the mise en pratique to
spell out the practical details. The explicit-constant definition in Mills et al. [17] for
the kelvin follows this approach:
(4) The kelvin, unit of thermodynamic temperature, is such that the Boltzmann
constant is exactly 1.380 65X X × 10−23 joule per kelvin.
After thorough discussions, the TG-SI is recommending the explicit-constant definition (4) because it is sufficiently wide to accommodate future developments and does
not favor any special primary thermometer for realizing the kelvin. Should the CCU
decide to adopt explicit-unit definitions for the kilogram, the ampere, and the mole,
then the second option of the TG-SI would be the formulation (1a) for the kelvin in
order to be in line with the other new definitions.
5 Progress of Experiments for Determination of the Boltzmann Constant
The value of the molar gas constant, R, recommended by CODATA in 2002 [11] is
essentially the weighted mean of two independent results for the speed of sound u 0
in argon obtained at a temperature close to and known in terms of the triple point of
water, TTPW . One result is from the National Institute of Standards and Technology
(NIST), USA [19], with a relative uncertainty u r = 1.8 × 10−6 , and the other from
the National Physical Laboratory (NPL), UK [20], with u r = 8.4 × 10−6 . Although
the two results of acoustic gas thermometry (AGT) are consistent, because of the large
difference in their uncertainties, the 2002 recommended value of R, and hence the
2002 recommended value of the Boltzmann constant k with u r (k) = 1.8 × 10−6 , is
to a very large extent determined by the NIST result.
In response to the recommendation T2 of the CCT [21], many projects have been
started to measure independently the value of the Boltzmann constant. Methods based on the thermal equation of state of ideal gases are shown in Fig. 1, including
their underlying basic equations [18]. These are, from left to right: the well-known
constant-volume gas thermometry (CVGT), acoustic gas thermometry (as discussed in
the previous paragraph and [22]), dielectric-constant gas thermometry (DCGT) using
audio-frequency capacitance bridges [23], measurement of n with refractive index gas
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Fig. 1 Principles of constant-volume gas thermometry (CVGT), acoustic gas thermometry (AGT),
dielectric-constant gas thermometry (DCGT), refractive index gas thermometry (RIGT), and thermometry
using quasi-spherical cavity resonators (QSCR) (γ0 = c p /cV is the ratio of specific heat capacities at
constant pressure and constant volume, M is the molar mass, ε0 is the dielectric constant, and α0 is the
static electric dipole polarizations of the atom) [18]
Table 2 Uncertainty in
determining the Boltzmann
constant by applying different
methods of primary
thermometry [18]
Method
AGT
DCGT
Present state (ppm)
2010 possibility (ppm)
2
1
15
2
TRT
32
5
QSCR
40
10
RIGT
300
30
DBT
200
10
thermometry (RIGT) applying optical resonators [24], and quasi-spherical cavity resonators (QSCRs) operating at gigahertz frequencies [25]. Other promising methods for
determining k are total radiation thermometry (TRT) [26,27], and Doppler-broadening
thermometry (DBT) [28,29].
Table 2 gives a summary overview of the potential of the currently-available primary thermometers for determining the Boltzmann constant k, as deduced from the
literature, a workshop held in 2005 at PTB [30], and recent information on new developments [18]. Table 2 illustrates that within the next four years there exists the possibility of achieving a reliable uncertainty of the value of k of the order of one part in 106
based on measurements applying different methods of primary thermometry. Thus,
an improved value of the Boltzmann constant proposed for defining the kelvin would
ideally have been determined by at least the two fundamentally different methods AGT
and DCGT and be corroborated by other—preferably optical—measurements such as
TRT and DBT with larger uncertainty.
The TG-SI appreciates the considerable progress of ongoing experiments to determine the Boltzmann constant in order to corroborate the present value. It is assumed
that the experiments currently underway to measure R or k will achieve consistent
results by the end of 2010, so that the CODATA group can recommend in its 2010
constants adjustment a new value for k with a relative standard uncertainty about a factor of two smaller than the current u r of approximately 2 × 10−6 . A value of u r (TTPW )
of about 1 × 10−6 , corresponding to about 0.25 mK, would reduce even more the
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insignificant differences between the thermodynamic uncertainties of columns 4 and
5 of Table 1. The TG-SI will continue to monitor the progress of new determinations
of the Boltzmann constant.
6 Conclusions
The Boltzmann constant is not connected with the other fundamental constants, in
contrast to its macroscopic counterpart, the molar gas constant R = k NA . Thus,
there are no alternatives to the linking of the kelvin aside from an exact value of the
Boltzmann constant.
Our recommendation is that the unit kelvin of temperature T should be defined
by fixing the value of the Boltzmann constant, thus proceeding in the same way as
with other units, with the aim to guarantee their long-term stability. The TG-SI is
proposing that the value of the Boltzmann constant to be taken for the redefinition is
as determined by the CODATA group in early 2011. For reasons of simplicity, our
preference is for an explicit-constant definition with accompanying text explaining
how the definition of the kelvin impacts upon primary and practical thermometry.
The new definition would be in line with modern science where nature is characterized by statistical thermodynamics, which implies the equivalence of energy
and temperature as expressed by the Maxwell-Boltzmann equation E = kT [31]. In
principle, temperature could be derived from the measurement of energy. In practice,
however, we have no simple and universal instrument for measuring energy and it
appears in different forms, e.g., temperature. The fundamental constant k converts the
value of this measurable quantity into energy units.
Also, the Consultative Committee for Electricity and Magnetism (CCEM) established a corresponding working group to study proposed changes to the SI, in response
to the recommendation of the CIPM [6]. The group organized a Round Table discussion on the proposed changes during the CPEM 2006 conference. Several experts
from national metrology institutes, and academia as well, expressed a favorable opinion about fixing the Boltzmann constant to redefine the kelvin [32]. The experts in
charge of studying the redefinition of the kilogram and the mole are also proposing
to redefine the kelvin so that it is linked to an exactly defined value of the Boltzmann
constant [17,31].
It is not always necessary that a new definition of a SI base unit should immediately
allow the unit to be realized with a reduced uncertainty [17]. The benefits to both
metrology and science of replacing the current definition of the kelvin by one that
links it to an exact value of the Boltzmann constant k, are viewed as outweighing
any marginal increase in the uncertainty of thermodynamic temperature that might
result. At very low and very high temperatures, there will be no need in the future
to reference back to the triple point of water, which the TG-SI considers as the main
practical advantage of the new definition.
As presented at the round-table discussion [32], the following time schedule for the
adoption of the new definitions could be envisaged: If the required experimental data
are available and in sufficient agreement in 2010, the values of the constants could be
chosen based on the 2010 CODATA constants adjustment (31 December 2010 closing
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date). The new definitions could be adopted by the 24th CGPM in October 2011.
Exact wordings of new definitions are to be developed by the CIPM through the CCU
in collaboration with the Consultative Committees and other interested parties. Mises
en pratique for the new unit definitions should be prepared by the CCM, CCEM,
CCT, and CCQM, and available by 31 December 2011. The TG-SI understands that
the updating of the mise en pratique for the definition of the kelvin [16] is under the
responsibility of the special task group formed by CCT WG1. The president of the
CCT will inform the other consultative committees of the work of this task group.
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